FIG. 8 is a cross-sectional view illustrating a conventional type of curvature variable reflecting curved-face mirror (a concave mirror), for instance, disclosed in Japanese Patent Laid Open Publication No. 159613/1986, and FIG. 9 is a perspective view thereof.
In the figure, designated at the reference numeral 1 is an air-tight vessel, at 2 a circular opening formed on the top edge thereof, at 3 a disk-shaped film with a peripheral section thereof fixed to the vessel 1, and at 4 a sealed tight space defined by the film 3 and the vessel.
Also, at 5 is a pressure adjusting means for adjusting a pressure in the sealed space 4, which comprises an air path 6 provided in the vessel 1, a valve 7 for opening/closing the path, and a pump 8 for discharging the air in the space 4.
In the curvature variable reflecting curved-face mirror (a concave mirror) constructed as described above, when the valve 7 is opened and the pump 8 is actuated, the air in the space 4 is discharged through the air path 6, so that the pressure inside of the vessel 1 becomes lower than that outside thereof. With this feature, a pressure difference is generated between the top and rear surfaces of the disk-shaped film 3, and for this reason this disk-shaped film 3 is deflected toward inside thereof.
Then the reflecting surface 3a which is an external surface of the film formes a substantially rotative paraboloid. For this reason, if an electromagnetic wave such as a light comes into this curved-face mirror from the above direction, the electromagnetic wave is focused to almost one point by the reflecting surface 3a. Namely, the mirror can be used as a concave mirror.
Then when a focus of the reflecting surface 3a reaches a prespecified position, an air flow inside and outside of the vessel 1 can be stopped by closing the valve 7, and a form of the disk-shaped film 3 can be kept constant.
Also if it is arranged so that air can be supplied from outside into the space 4 of the vessel 1 by the pump 8, the pressure in the vessel 1 can be made higher than that outside thereof, so that the reflecting film 3a can be deflected toward outside thereof. For this reason, a convex mirror can be formed therein.
The curvature variable reflecting curved-face mirror based on the conventional technology is constructed as described above, and for this reason it is required for the purpose to change the curvature to change the pressure inside of the vessel 1 by actuating the pump 8, and a certain time is required until the curvature becomes as prespecified, so that, for instance, in a light scan type of laser machining apparatus in which a machining head moves on an X-Y plane and executes machining, in a case where the curvature variable reflecting curved-face mirror described above is used for reflecting a laser beam, it has been difficult to keep constant a diameter of a laser beam coming into a converging optical member by changing a diameter of laser beam at a high speed in association with movement of the machining head.